In recent years, there has been a growing concern to the conservation of global environment, and regulations against over-fertilization of sea areas have been enforced. Thus, the development of a new technology for removing nitrogen from an effluent has been sought. In answer to such request, the removal of the nitrogen contained in an effluent has been conducted from some time ago mainly by the following methods:    1) Biological denitrification method: Method in which an organic nitrogen contained in water is converted into an inorganic nitrogen to render the organic nitrogen harmless by using a bacterium.    2) Discontinuous NH3 decomposition method with chlorine: Method in which NH3 is oxidized to decompose by using sodium hypochlorite.    3) Ion exchange method: Method in which NH3 is adsorbed on a zeolite through an ion exchange.    4) Ammonia stripping method: Method in which NH3 is diffused or evaporated from an NH3-containing effluent into the atmosphere by using air or steam.
When the BOD (biochemical oxygen demand) of an effluent is high, biological denitrification method 1) described above is used. On the other hand, in the case where an effluent in which most of nitrogen is in a form of ammonia nitrogen such as ammonia and ammonium ion is to be treated, for instance, when an effluent from a process in a chemical factory or an effluent once-subjected to a post-treatment is the object of the treatment, method 2), 3) or 4) is used.
However, the conventional methods described above have the problems as follows:
In the method 1), the size of a reaction bath necessary for the treatment becomes large since the rate of a biological reaction is slow, and thus a large space becomes necessary for placing the reaction bath. Besides, the method 1) raises the problem that excess amount of a sludge is produced. Method 2) causes the problem that a treatment of remaining chlorine becomes necessary and organic chlorine compounds are formed, since the addition of sodium hypochlorite in an amount more than that stoichio-metrically required is necessary for completely removing the ammonia. In the method 3), a secondary effluent containing ammonium ion at a high concentration is produced at the time of regenerating a used zeolite and thus a treatment of the secondary effluent becomes necessary. Further, the method 4) has the problem that an NH3-containing gas is diffused or dissipated into the atmosphere after the NH3 was transferred into a gas phase and causes a secondary pollution.
Among the methods described above, method 4) is advantageous compared with other methods since the treatment of an effluent is comparatively simple and the costs of equipments and operations are small. Accordingly, a combination in which the method 4) is performed in combination with another method which can be used for oxidizing to decompose the NH3 contained at a high concentration in a gas separated from an effluent, by using a catalyst, to make the NH3 contained in the effluent harmless as the result of the combination has been adopted even in current night-soil treatment facilities. However, in such a stripping and catalytically oxidizing process, it is necessary to install a catalyst tower for reducing NOx in addition to a catalyst tower for oxidizing NH3 since a large quantity of NOx is generated at the time of the oxidation of NH3. Further, according to the investigations by the present inventors, it has been found out anew that a large quantity of N2O is also produced in this process at the time of oxidizing the NH3. Like CO2, N2O is a substance contributing to the global warming. Accordingly, it is dangerous to the global environment that a large quantity of N2O is diffused into the atmosphere, in the same extent as NH3 is diffused as it is. Thus, the diffusion of N2O is also undesirable.
As described above, treatments of NH3-containing effluents in conventional technology have many problems and some of the treatments had a problem that they might become sources from which various secondary pollution substances are produced anew.